Incorrect (imperative updates):

// app/components/shopping-cart.gjs
import Component from '@glimmer/component';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

class ShoppingCart extends Component {
  @tracked items = [];
  @tracked subtotal = 0;
  @tracked tax = 0;
  @tracked shipping = 0;
  @tracked total = 0;

  @action
  addItem(item) {
    this.items = [...this.items, item];
    this.recalculate();
  }

  @action
  removeItem(index) {
    this.items = this.items.filter((_, i) => i !== index);
    this.recalculate();
  }

  recalculate() {
    this.subtotal = this.items.reduce((sum, item) => sum + item.price, 0);
    this.tax = this.subtotal * 0.08;
    this.shipping = this.subtotal > 50 ? 0 : 5.99;
    this.total = this.subtotal + this.tax + this.shipping;
  }

  <template>
    <div class="cart">
      <div>Subtotal: ${{this.subtotal}}</div>
      <div>Tax: ${{this.tax}}</div>
      <div>Shipping: ${{this.shipping}}</div>
      <div>Total: ${{this.total}}</div>
    </div>
  </template>
}